The present invention relates to a method for processing the output signal of a rotational speed sensor which is present in the form of an alternating voltage whose frequency and amplitude are a function of the rotational speed. Using the method in an input circuit, the high-frequency interfering signals are attenuated and the useful signal is amplified and transformed into a pulse-shaped output signal. Circuit configurations with low-pass filters for the attenuation of high-frequency interfering signals as well as with amplifying and pulse-generating circuits are utilized in the present invention.
Methods and circuit configurations of this type are know which are used in automotive vehicles with electronic brake slip control (German published patent application No. 32 34 637). In such brake systems, the information about the wheel rotational behavior required for the control is obtained by means of the rotational speed sensors. To this end, for instance a disc co-rotating with the wheel or with the drive shaft is used, which disc is toothed at its periphery and which coacts with a stationary inductive pick-up for measuring data which issues an alternating voltage having a frequency proportional to the wheel rotational speed. This is because owing to the toothing of the ferromagnetic toothed disc passing by the sensor, a magnetic flux generated by a permanent magnet is varied periodically, with the result that an alternating voltage is induced in the measuring coil of this sensor, the frequency of which is proportional to the velocity of the wheel. The sensor signals will then be processed in a trigger circuit, that is amplified and transformed into a binary square-wave signal. Additionally, the trigger circuits contain filters by which interfering signals are attenuated to the greatest extent possible.
When using such measuring arrangements, difficulties arise in that the amplitudes of the induced voltages and, consequently, the output voltages of the sensors are dependent on the rotational speed and become very weak in the presence of slow vehicle speed and wheel speed. In order to minimize the high-frequency interfering signals as well as the low-frequency voltage fluctuations caused by dividing errors of the gear wheel, eccentricity of the wheel or eccentric support of the wheel etc. to such extent as to manage in the evaluation to obtain an isolation form the useful signal, that means the speed-responsive signal, a major expediture is required. For example, the conduit leading from the rotational speed sensor to the electronic processing circuit must be bridged by shielded lines. The mechanical tolerances of toothed disc and sensor must be limited to the still admissible valves by finishing and adjusting operations. This increases the manufacturing effort significantly because at least three or four sensors are required for precise brake slip control.
Therefore, it is an object of the present invention to overcome the described shortcomings in measuring the rotational speed and processing the measuring signals and to device a method as well as circuit configurations which permit processing even relatively weak sensor signals affected by high-frequency and very low-frequency interferences. That is, exact distinction between useful signals and interfering signals must be rendered possible even under such unfavorable conditions as prevailing in particular at a low rotational speed.
This object is achieved in a surprisingly simple fashion in that, according to the instant invention, in a method of the type referred to, a reference signal is formed in the input circuit by means of a low-pass filter, with the limit frequency being below the useful frequency range, in that the useful signal is compared with the reference signal and in that, as a function of the difference between these two signals, the pulse-shaped output signal is generated which represents the processed sensor signal.
The special design of a circuit configuration for implementing the method resides in a smoothing filter which is inserted into the input circuit, namely between the rotational speed sensor and the amplifying and pulse-generating circuits, which smoothing filter contains the low-pass filter for attenuating the high-frequency interfering signals and forming the useful signal, as well as a low-pass filter for obtaining a reference signal, and in that the signals obtained by means of the two low-pass filters, namely the useful signal and the reference signal, can be supplied to a comparator whose output signal represents the processed sensor signal.
The advantages of the present invention, when used for a slip-controlled vehicular brake system, are above all the reduced expenditure for measuring the wheel speeds and for transmitting this information to the associated electronic control unit. As the present invention permits unambiguous distinction between the useful signal representative of the rotational speed even in the presence of a low amplitude and the high-frequency interfering signals and the voltage fluctuations due to eccentricities etc., comparatively simple rotational speed sensors will suffice which, moreover, admit greater manufacturing and assembling tolerances. Instead of shielded lines, unshielded lines may be used to connect the sensors to the electronics. The results of all this are considerably less manufacturing efforts as well as improved functioning.
According to an expedient embodiment of the inventive method, when forming the digital output signal, a hysteresis will be created between the control signal, that is the difference between the useful signal and the reference signal, and the change-over of the output signal.
Another important embodiment of this invention resides in that the reference signal, as soon as the difference between the useful signal level and the reference signal level exceeds a predetermined threshold value, will dynamically follow the useful signal. This is of particular significance for the dimensioning of the filters in the input circuit what will be explained in more detail hereinbelow.
The limit frequency of the low-pass filter for the attenuation of the interfering signals and for the formation of the useful signals suitable is within the effective frequency range and is chosen such that, above the limit frequency, the amplitudes of the useful signals which rise at the output of the rotational speed sensor proportional to the rotational speed are approximately the same.
In another embodiment of the inventive circuit configuration, the useful signal and the reference signal can be delivered to the two inputs of a comparator. Expediently, impedance transformers are inserted between the low-pass filters and the comparator.
Another embodiment of the instant invention resides in that a resistor is interposed in the signal lines between the impedance transformers and the comparator, on which resistor a difference voltage can be generated by means of a difference amplifier controlled by the output signal of the comparator and by means of a current mirror circuitry, which difference voltage is opposed to the actuating signal of the comparator, i.e. the difference between the useful signal and the reference signal, and thereby causes the switching hysteresis.
Furthermore, the reference signal can be variable as a function of the amplitude of the useful signal. By means of an adapter circuit which starts to function after a predetermined threshold value, that is the difference value between the useful signal level and the reference signal level, has been exceeded, the reference signal can dynamically follow the useful signal. To generate the difference threshold value decisive for the follow-up, a resistor can be inserted into the signal line leading to the adapter circuit, on which resistor a threshold voltage opposing the useful signal can be generated by means of a current mirror circuitry and a difference amplifier controlled by the output signal of the comparator.
The adapter circuit may comprise a proportional amplifier which, by way of a network connected downstream, generates a control variable which is dependent on the difference between the useful signal level and the reference signal level and which can be fed back to the low-pass filter circuit forming the reference signal.
According to another embodiment of the inventive circuit configuration, for the self-monitoring of its operability, there is provision of a supplement circuit which initiates inherent oscillation of the evaluating circuit with a frequency that is below the useful frequency range. Upon the occurrence of a short-circuit or interruption of the signals leading to the rotational speed sensor, the supplement circuit will stop the inherent oscillation.
The supplement circuit, expediently, possesses a counter constantly stepped by means of an oscillator or a clock generator, which counter can be reset by the output signal of the comparator and which, on attainment of a predetermined digit, by way of pulse generator will introduce a pulse similar to a useful signal into the input circuit and thereby will initiate a pulse at the output of the comparator that resets the counter. This way, the entire trigger circuit will be dynamically checked for operability as long as the vehicle stands still and no wheel sensor pulse is generated.
Besides, to monitor the circuit configuration according to the present invention a constant current source may be provided which causes a distribution of voltage in an input network which, in the event of interruption of the line leading to the rotational speed sensor or a short-circuit, exceeds limit values and thereby stops the inherent oscillation and/or the input of pulses into the input circuit which causes the inherent oscillation.
Hence, it follows that the inventive supplement circuit results in discontinuance of the inherent oscillation and thus in an error indication both on the occurrence of a short-circuit in the sensor circuit and a defect in the evaluating circuit. No additional signal lines or other auxiliary means are required for this purpose, which therefore represents a major progress compared to know systems of this type.